In the production of microelectronic devices, especially printed and integrated circuitry, it is necessary to conduct a series of process steps at predefined points on the surface of the device. The pattern of movement of test probes and wire bonding tools, or other process apparatus, over the surface to reach those defined points is repeated in the case of each device. It is current practice to automate the process tool movement in mass producing such devices. Other steps in the production process are also automated, including the steps in which such "predefined points" are produced. It is not unusual that manufacturing variations result in displacement of the component, including those process points, and that gives rise to the need for some means to detect and overcome displacement errors so that automatic processing can continue.
An example of the problem is found in the manufacture of integrated circuit devices where the circuit is formed on a substrate, the substrate is mounted on a base, and the base is mounted in or on a housing in common with external connector elements. A device constructed in that fashion is illustrated in FIG. 1 of the accompanying drawing. A fourteen-pin integrated circuit device is shown in a partially completed "DIP" package. The circuit includes a number of wire bonding pads which are connected by fine conductor wires to the inner ends of respectively associated connector pins. Those wires are added to the unit by a bonding machine that bonds an end of a wire to a pad in the circuit and leads the wire to the inner end of one of the connector pins. The machine bonds the wire to that connector and then severs the standing part of the wire. That process is repeated over and over until all of the wire connectors are completed one at a time.
The minimum wire size, and the smallest bonding tools, that can be used successfully in the current state of automated integrated circuit production, results in bonds whose areas cover several square mils. As designers add more and more elements to integrated circuits, and attempt to fit them into existing packages, the area allocated to bonding pads is reduced. Pad area may be only a few square mils. The fact that the bond area approaches the pad area poses a problem if the pad is displaced from its assigned position. If the pad is displaced by distances in the order of 1 or 2 mils, the automatic bonding process may produce defective bonds and may even produce short circuits and improper connections to circuit points adjacent to the bonding pads.
The process by which printed and integrated circuits are produced results in a high degree of uniformity in the surface geometry between the individual pieces resulting from continuous or lot production. Thus, little variation is found in the relative positions of circuit pads and test points on integrated circuit substrates or chips. However, it is not at all uncommon to find variations in placement of the circuit on the substrate, in placement of the substrate on the base, and in placement of the base relative of the connector pins. That there is ample opportunity for such variation will be apparent from an examination in FIG. 1.
Placement error can be categorized as lineal translations and angular displacements common to all of the test and bond points of the circuit. Displacement errors which involve no angular displacement and are limited to lineal translation are relatively easy to overcome. In such a case, the direction and amount of correction required at any one process point of the circuit is required at all process points of that circuit. It is usually a relatively simple matter to alter an automatic processing program to apply a uniform correction when locating process points. It is necessary to interrupt automatic processing to measure displacement at one point, but once that has been accomplished, processing can be automatic.
Correcting angular displacement is not so simple. The amount of lineal displacement at each process point, as a consequence of rotational translation, is a function of the position of the center of rotational displacement and of the angle of displacement. The difficulty is greatly compounded when rotational displacement is combined with translational displacement.
Errors can be calculated and compensated for if the direction and the amount of displacement is measured at two or three process points. Trigonometry and arithmetic and geometry can be used to calculate the amount of translation and the degree of rotational displacement. Having that information, trigonometry, arithmetic and geometry can be used to calculate the correction required at each process point. The calculation can be accomplished using a computer. However, it is required to employ trigonometric functions and identities and to employ geometric formulas. Trigonometric tables must be stored or computed and identities and formulas must be stored or derived. Further, the computation of each process point may require a number of conversions from rectangular to polar coordinates and back again. The result is slow computation and slow processing, unless a large computer is employed. Large computers are expensive and it is not uncommon to resort to time sharing techniques and the complication they introduce to permit the employment of greater computer power.